Catalytic cracking is a petroleum refining process that is applied commercially on a very large scale. Indeed, fluidized catalytic cracking (FCC) processes produces a large amount of the refinery gasoline blending pool in the United States. In the process, heavy hydrocarbon feedstocks are converted into lighter products by reactions taking place at elevated temperatures in the presence of a catalyst, with the majority of reactions taking place in the vapor phase. The feedstock is thereby converted into gasoline, distillates and other liquid fraction product streams as well as lighter gaseous cracking products having four or less carbon atoms per molecule. The three characteristic steps of a catalytic cracking process comprises: a cracking step in which the heavy hydrocarbon feed stream is converted into lighter products, a stripping step to remove adsorbed hydrocarbons from the catalyst material, and a regeneration step to burn off coke formations from the catalyst material. The regenerated catalyst is then recirculated and reused in the cracking step.
Catalytically cracked feedstocks normally contain organic sulfur compounds, such as mercaptans, sulfides, thiophenes, benzothiophenes, dibenzothiophenes, and other sulfur-containing species. The products of the cracking process correspondingly tend to contain sulfur impurities even though about half of the sulfur compounds are converted to hydrogen sulfide during the cracking process, mainly by catalytic decomposition of non-thiophenic sulfur compounds. The thiophenic compounds have been found to be most difficult to remove. The specific distribution of sulfur in the cracking products is dependent on a number of factors including feed, catalyst type, additives present, conversion and other operating conditions, but, in any event a certain proportion of the sulfur tends to enter the light or heavy gasoline fractions and passes over into the product pool. Although petroleum feedstock normally contains a variety of sulfur born contaminants, one of the chief concerns is the presence of unsubstituted and hydrocarbyl substituted thiophenes and their derivatives, such as thiophene, methylthiophene, ethylthiophene, propylthiophene, tetrahydrothiophene, benzothiophene and the like in the heavy and light gasoline fraction product streams of FCC processes. The thiophenic compounds generally have boiling points within the range of the light and heavy gasoline fractions and, thus, become concentrated in these product streams. With increasing environmental regulation being applied to petroleum products, for example in the Reformulated Gasoline (RFG) regulations, there has been numerous attempts to reduce the sulfur content of the products, especially those attributable to thiophenic compounds.
One approach has been to remove the sulfur from the FCC feed by hydrotreating before cracking is initiated. While highly effective, this approach tends to be expensive in terms of the capital cost of the equipment as well as operationally since hydrogen consumption is high. Another approach has been to remove the sulfur from the cracked products by hydrotreating. Again, while effective, this solution has the drawback that valuable product octane may be lost when the high octane olefinic components become saturated.
From an economic point of view, it would be desirable to achieve thiophenic sulfur removal in the cracking process itself since this would effectively desulfurize the major components of the gasoline blending pool without additional treatment. Various catalytic materials have been developed for the removal of sulfur during the FCC process cycle. For example, an FCC catalyst impregnated with vanadium has been shown to reduce the level of product sulfur (See U.S. Pat. No. 6,482,315). This reference also discloses a sulfur reduction additive based on a zinc-impregnated alumina.
Other developments for reducing product sulfur have centered on the removal of sulfur from the regenerator stack gases. An early approach developed by Chevron used alumina compounds as additives to the inventory of cracking catalyst to adsorb sulfur oxides in the FCC regenerator; the adsorbed sulfur compounds which entered the process in the feed were released as hydrogen sulfide during the cracking portion of the cycle and passed to the product recovery section of the unit where they were removed (See Krishna et al., Additives Improved FCC Process, Hydrocarbon Processing, November 1991, pages 59-66). Although sulfur is removed from the stack gases of the regenerator, product sulfur levels are not greatly affected, if at all.
An alternative technology for the removal of sulfur oxides from regenerator stack gases is based on the use of magnesium-aluminum spinels as additives to the circulating catalyst inventory in the FCC unit (FCCU). Exemplary patents disclosing this type of sulfur removal additives include U.S. Pat. Nos. 4,963,520; 4,957,892; 4,957,718; 4,790,982 and others. Again, however, sulfur content in liquid products, such as gasoline, was not greatly affected.
A catalyst composition to reduce sulfur levels in liquid cracking products has been described by Wormsbecher and Kim in U.S. Pat. Nos. 5,376,608 and 5,525,210. These patents propose the addition of low amounts of an additive composed of an alumina-supported Lewis Acid to conventional zeolite-containing cracking catalyst. Although this system has the advantages of causing sulfur reduction in the cracking process, it is generally believed that use of greater than about 10 weight percent of the described additives in the catalyst composition does not provide a benefit (e.g. high sulfur removal while retaining the selectivity of other products) proportional to the level of the additive. In view of the fact that an FCCU can only contain a fixed amount of fluidized particulates, the inclusion of additives, such as the alumina-supported Lewis Acid additives of Wormsbecher and Kim, causes a reduction in the amount of the base cracking catalyst contained in the FCCU and thus, a proportional reduction in the conversion of heavy feedstock to desired products.
U.S. Pat. No. 6,635,168 discloses a FCC catalyst composition composed of Lewis Acid-containing alumina and Y-type zeolite containing catalyst to provide a composition having a kinetic conversion activity of at least 2. This product was developed in part to address disadvantages associated with the aforementioned Lewis Acid components. Indeed, the compositions described in U.S. Pat. No. 6,635,168 provide a reduced sulfur (e.g., thiophenes and derivatives thereof) content in light and heavy gasoline fractions of the FCC processes, (about 34%).
Sulfur standards are becoming more stringent as evidenced by the fact that the U.S. Environmental Protection Agency has set new standards for gasoline sulfur content and is reducing the average from the current standard of 350 ppm sulfur to about 30 ppm by 2006. It therefore would be desirable to have a catalyst composition suitable for use in FCC processes wherein the catalyst is capable of significantly reducing the level sulfur even further than those achieved using additives described in WO 02/08300, especially reducing the level of thiophenes, and their derivatives from light and heavy gasoline fractions while substantially retaining conversion of feedstock to desired product, e.g., substantially reducing the levels of thiophene and its derivatives as part of the functions of the FCC process while substantially maintaining the overall cracking activity and product selectivities. It is also desirable to have an additive possessing sulfur reduction activity that does not substantially degrade in relatively short periods of time, i.e., the additive's sulfur reduction activity is maintained over longer periods of time.